This invention relates to improvements in the separation of gas mixtures, and more particularly to a process for the separation of gas mixtures by selective adsorption.
Gas mixtures having selectively adsorbable components can be separated by certain adsorbent materials, and this technique for the separation of gases is generally known as the pressure swing separation of gases. Commercially available adsorbent materials with selective adsorption characteristics are well known for these processes. Each adsorbent has unique characteristics which adapt its use to various gas separation systems. The various applications for such systems range from the separation of complex organic and/or inorganic gas mixtures to the purification of air by the removal of water and carbon dioxide. The prior art is replete with examples of these mixtures of gases which can be separated by the pressure swing processes. For example, ethane, propane, ethylene or propylene can be separated from each other or from higher gaseous paraffins or olefins; sulfur oxides, hydrogen sulfide, carbon dioxide, carbon disulfide and/or carbonyl sulfide can be removed from natural gas, ethane, propane, butane, ethylene, propylene, isoprene or butadiene; and carbon dioxide and/or nitrogen can be separated from air so as to purify the air or provide an oxygen enriched air. Although the present process is specifically described and illustrated in relation to the application of pressure swing adsorption to the fractionation of air as a means of producing an oxygen rich stream, it is broadly applicable to the separation of organic and/or inorganic gas mixtures.
There have been many pressure swing adsorption systems set forth as a means of separating air into basically an oxygen rich stream and a nitrogen rich stream. The oxygen rich stream is of greatest commercial interest due to its many and varied uses. The main goal of these systems has been to conserve oxygen and thereby obtain the highest possible oxygen recovery from the feed air stream. This has been accomplished by utilizing one or more adsorbent beds and on occasion, one or more empty storage tanks connected and sequentially arranged such that a minimum low purity oxygen stream is vented to the atmosphere while a maximum of high purity oxygen is available as a product oxygen stream.
One of the pressure swing adsorption systems is described by Marsh et al in U.S. Pat. No. 3,142,547. Marsh et al provide a cyclic scheme of alternately diverting lower pressure product oxygen from either one of two adsorbent beds for storage in an empty tank for later use as a countercurrent purge gas for the low pressure desorbing bed. This involves the preparative repressurization of the non-adsorbing bed with product oxygen from the adsorbing bed prior to switching the feed stream to the purged repressurized bed. However, it is limited in quantity of void gas recovery to that which can be blown down before pressure equalization occurs between the adsorption bed and the surge tank. Moreover, it diverts a low purity oxygen stream to the atmosphere and delivers product quality oxygen only after the maximum adsorption pressure has been reached.
U.S. Pat. No. 3,738,087 to McCombs et al describes several cycles wherein repressurization occurs partially with feed gas after initial bed pressure equalization step(s). Product quality oxygen is removed from the bed being repressurized with feed air in McCombs et al, this being referred to as variable pressure adsorption.
In U.S. Pat. No. 3,788,036, Lee et al describes a sequential pressure equalization technique where high pressure in the adsorbent bed which is to commence a regeneration phase is conserved by a dual pressure equalization. Lee et al conserve oxygen to a greater extent than McCombs and Marsh et al by adding an empty tank to the system and using this tank to supply the purge gas to the depressurized desorbing bed and showed significantly improved performance over Marsh et al and McCombs, but Lee et al cannot deliver continuous product oxygen without adding a product surge tank, nor can Lee et al continuously receive an uninterrupted flow of feed air.
Other pressure swing adsorption processes are also described in the prior art. However, these systems also have the same shortcomings as described supra and/or require four or more adsorbent beds with concomitant piping and valving to provide efficient separation of gases, uninterrupted flow of product gas and/or continuous flow of feed air stream.